1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive film type device and particularly to a piezoelectric/electrostrictive film type vibrator which is commonly employed in a microphone or a sensor for sensing the fluidity, the acoustic pressure, the weight of small level, the acceleration, and so on.
2. Description of the Related Art
Piezoelectric/electrostrictive film type devices have been used as actuators or sensors. One of such piezoelectric/electrostrictive film type devices used as a sensor is disclosed in Japanese Patent Laid-open Publication (Heisei)8-201265 which is utilized for measuring the density, the concentration, and the viscosity of a fluid. Examples of the fluid to be measured are liquid and gas, including not only single substance liquid such as water, alcohol, and oil but also solution, slurry, and paste where a soluble or non-soluble medium is dissolved, mixed, or suspended in the liquid. The device acts as a sensor utilizing the correlation between the amplitude of a piezoelectric vibrator and the viscous resistance of the fluid to the vibrator. A vibrating motion of the mechanical system of the vibrator can be replaced by an equivalent circuit of the electric system. More specifically, when the piezoelectric/electrostrictive film type vibrator vibrates in and receives a mechanical resistance from the fluid due to its viscosity, its equivalent circuit generates a change in the electrical constant which is then sensed to determine the viscosity, the density, and the concentration of the fluid. The electrical constant may be selected from loss factor, phase, resistance, reactance, conductance, susceptance, inductance, and capacitance. Particularly, the loss factor or phase which exhibits a maximum or minimum change point near the resonant frequency of the equivalent circuit may preferably be used as an index factor. This enables the measurement of not only the viscosity but also the density and the concentration of the fluid. For example, the concentration of sulfuric acid in a sulfuric acid solution may successfully be detected. It would be understood that the index factor indicative of a change in the vibrating state is not limited to the electrical constant but may be a change in the resonant frequency unless. otherwise specified in view of the measuring accuracy and the durability.
Such a piezoelectric/electrostrictive film type vibrator disclosed in Japanese Patent Laid-open Publication (Heisei)5-267742 has an auxiliary electrode 8 provided independently of the lower electrode 4 on a substrate 1 which is made of a ceramic material and includes a thin diaphragm 3 surrounded by a thick region 2 as shown in FIG. 2. A portion of the auxiliary electrode 8 is located beneath a piezoelectric/electrostrictive film 5. The upper electrode 6 is provided extending continuously over the auxiliary electrode 8 and the piezoelectric/electrostrictive film 5 and thus the connecting reliability is improved. In FIG. 2, the fluid to be measured is contained in a cavity 10 as having been introduced from through holes 9.
It is essential for such a sensor device which detects an electrical change during the vibration not to fluctuate in electrical constant. However, its common piezoelectric/electrostrictive film type device structure for sensing the characteristics of a fluid including the acoustic pressure, the weight of small level, and the acceleration may exhibit variations of the electrical constant at the initial stage between the devices and produce a negative change with time. This requires a troublesome action of readjusting the sensor device.
We have found through a series of experiments that, as shown in FIG. 2, the conventional piezoelectric/electrostrictive film type device for a sensor has the lower electrode 4 arranged at its auxiliary electrode 8 side to extend along the diaphragm 3 and the auxiliary electrode 8 is located on the thick region 2 but not on the thin diaphragm 3, hence allowing a region of direct contact between the piezoelectric/electrostrictive film 5 and diaphragm 3 and generating an incompletely bonded region 7 over the thin diaphragm 3 and the thick region 2 due to chemical reaction and so on. It is proved for the sensor device which utilizes detection of the electrical constant during vibration, that variation or degradation with time of the incomplete bonding state at the incompletely bonded region 7 is one of the major causes for varying the vibrating motion or the electrical constant. Therefore, a piezoelectric/electrostrictive film type device according to the present invention which has a lower electrode, an auxiliary electrode, a piezoelectric/electrostrictive film, and an tipper electrode provided in layers on a substrate made of a ceramic material having a this diaphragm thereof surrounded by a thick region is provided wherein the lower electrode is arranged extending continuously from the thin diaphragm to the thick region, the auxiliary electrode is provided on the thick region at a location separated from the lower electrode, and the piezoelectric/electrostrictive film is arranged bridging between the lower electrode and the auxiliary electrode. This allows the incompletely bonded region between the piezoelectric/electrostrictive film and the substrate to be located off the thin diaphragm, thus significantly minimizing variations of the vibrating motion.